Process of recovering coal fines



Unite. T States Patent 3,070,331 Patented Feb. 26, 193

free

3,079,331 PRGCESS F RECI'JVERENG (IQAL FENEF; Eimer W. Gieseke, Fountain(Ilty, Term assignor to American Cyanarnid Company, New York, N.Y., a

corporation of Maine No Drawing. Filed Aug. 27, E59, Ser. No. 835,338

' 2 Claims. (Cl. 210-53) This invention relates to a process ofrecovering naturally flotatable coal fines. More specifically, itrelates to a process of recovering finely divided coal which isnaturally flotatable, which process comprises the successive steps ofadding to a slurry of such coal fines a re agent such as soluble ligninsulfonates, dextrin, sulfonated nigrosines or mixtures thereof, thendecanting the slurry from such coal as settles, adding to the decantedslurry a flocculating agent consisting of a mixture of a Water solublepolyacrylic polymer and pregelatinized starch and decanting the liquidfrom the coal fines.

In the processing of coal, there is inevitably formed some very finelydivided coal. Such finely divided coal passes on through the wet sievingprocess as a slurry. Since there is a considerable quantity of suchcoal, effonts are made to recover as much as possible. This recovery iscomplicated by the fact that powdered coal is often naturally flotatableand furthermore is often contaminated with oils during the miningoperation, which increases the tendency to float. Consequently, thisfinely divided coal does not settle readily in the tanks to which suchslurries are run. Often some of the coal forms in a froth on top of thetank and will not settle. In the past the froth of unsettled finelydivided coal has been run into a second tank in which starch and limeare added in an effort to induce settling. However, because of thetendency of the slurry to froth, while some was recovered by thismethod, the recovery has not been satisfactory and some coal finesalways remain slurried in the efiluent liquor. Such losses not only arein economic waste but cause considerable pollutions of streams.

A further disadvantage of the prior operations was in fact that thefroth, which was formed from the finely divided coal not only cannot besettled but similarly cannot be filtered when formed in a flotationprocess. Thus this frothing interfered with both settling byflocculation and with the recovery of such fines by a flotation process.

I have found first that the frothing of finely divided coal can bereduced and even eliminated, by the addition to the aqueous slurry ofsoluble lignin sulfonates, dextrin, sulfonated nigrosines and mixturesthereof, and that, with the froth thus eliminated, a certain amount offurther settling occurs from such fines and the remaining slurry can beeither filtered or flocculated for filtration. I have further found thatthe flocculation of such a defoamed slurry can be effectively andeconomically carried out by adding thereto a mixture of a water solublepolyacrylic polymer and pregelatinized starch.

The coal fines which are recoverable by the process of my invention arethose which naturally float in water and which do not settle readily. Inthe processing of coal the minus A" screen sized coal is normallydeslimed in large bowl desilters, drag tanks or similar devices. Much ofthe coal floats as a froth product over these desliming devices andnormally goes to the thickeners where flocculation agents are usuallyadded to settle the solids. A great deal of material still normallyremains floating and is pumped to Waste. in the process of my inventionthe defoaming reagent is placed in the desilming device and theflocculation agent is usually added to the solution in the thickenertanks, although I can perform both operations in the same tank ifdesired.

The defoaming reagents which may be used in the first step includeswater soluble lignin sulfonates such as the sodium, potassium or calciumsalts, dextrin, sulfonated nigrosines and water soluble salts thereofand various mixtures thereof. In order to destroy the froth it isnecessary to add at least 0.0005 to 0.001 part of these reagents perparts of the water in the slurry.

The flocculating agents which can be used in conjunction withpregelatinized starch to obtain effective flocculation of the slurry ofcoal fines include polyacrylic polymers of greater than one hundredthousand molecular weight. The polyacrylic polymer can bepolyacrylamides, hydrolyzed polyacrylonitriles, copolymers of acrylamideand acrylic acid and the like. These polymers having acrylic acid areused in the form of the sodium salt. Preferably molecular weights shouldbe greater than 300,000 and the best results are obtained with evenhigher molecular weights such as 2,000,000 or even higher. In additionone can use copolymers of vinylacetate and maleic anhydride in the formof the sodium salt, when molecular weights of the above ranges areachieved. These polymers are used in conjunction with pregelatinizedstarch. At least 0.25 to 10 parts per million parts of water in theslurry of the combined polymer and starch must be used and the polymerto starch ratio must be 1:20 to 1:0 (i.e. the mixture is 5 to 100%polymer).

The coal fines thus fiocculated are recovered by filtration and dried.

The process of my invention is usable in the recovery of coal fines fromthe tailings from the processing of coal, either at the desliming stageas described above or in the waste effluent from such coal processingplants after more conventional treatment has been used. Normally thewaste products from such coal processing plant are passed into what isknown as a drag tank in order to permit the final settling of all wastesolids. Even here normally the coal fines pass on through into theetfluent from the plant. The use of the foam regulating reagent plus theflocculation mixture in such drag tanks effectively prevents the passageof coal fines into the river or other depository of waste liquor.

One advantage of my invention is the economic one that a large amount ofcoal which otherwise passes out unrecovered and is lost can now berecovered. Another advantage of my invention is that the foaming which,in processing plants handling large quantities of such slurries, canbuild up very badly in the thickening tanks has been eliminated. Attimes the foam builds up in the thickening tanks to the point where itis neccssary to shut the plant down, a process which can be quiteuneconomic. This is eliminated by the use of the foam regulatingcompounds as described above.

My invention can be illustrated by the following examples.

Example 1 At one large coal processing plant the minus one quarter inchscreen sized coal is normally deslimed in eight forty four feet diameterbowl desilters. Much of the coal overflows as a froth product and asfines over these desilters and normally goes to two foot diameterthickener tanks where an effort is usually made to settle the solids.The solids recovered in these thickeners is fine coal and normallyamounted to about 120 tons per hour.

To the desiltcr tank discharge is added 5 lbs. per hour per each 1,000gallons per minute of slurry of a mixture containing 70% crude calciumlignin sulfonate, 29.4% corn dextrin and 0.6% sulfonated nigrosine. Theaddition is made to the overflow as it flows out of the desilter intoone of the thickening tanks. 60 tons per hour of fine coal is recoveredin this thickener as underflow and the overflow was run into a secondthickener where the fiocculating agents, namely a mixture of 0.25 to 0.5part of polyacrylamide of approximately 2,000,000 molecular weight and2.5 to 5.0 parts of pregelatinized starch per million parts of slurry isadded. The overflow from the second thickener is sufficiently clear thatit can be reused in the plant. The total recovery of coal amounts toabout 60 tons per hour greater than obtained in that plant when notusing the process of my invention.

Example 2 The procedure of Example 1 is followed using as the defoamingreagent the various following reagents:

( 1) 100% crude calcium lignin sulfonate.

(2) A mixture of 100 parts of dextrin and 200 parts of nigrosine.

(3) A mixture of 50% crude calcium lignin sulfonate with 50% sulfonatednigrosine.

The results in each case are similar to that obtained in Example 1.

Example 3 A coal processing plant which runs its eflluent into a dragtank to settle out all solids found a considerable quantity of fine coaldust passing out into the nearby stream. To the drag tank overflowing atthe rate of 1000 gallons per minute is added 10 lbs. per hour of aslurry of the foam regulating composition described in Example 1followed by 1 to 2 lbs. per hour of a mixture consisting of 1 part ofpolyacrylamide of molecular weight about 2,000,000 and parts ofpregelatinized starch. As a result the overflow from this drag tank isperfectly clear.

Example 5 The procedure of Example 4 is followed except that 4 theflocculation reagent is 1 to 2 pounds per hour, of polyacrylamide ofmolecular Weight about 2,000,000, no starch being used. The overflowcomes clear.

I claim:

1. A process of recovering floating coal fines from aqueous slurriestopped with coal fine-containing-froths which comprises the successivesteps of (1) adding to said slurries about 2.5 to 10 parts per millionof a defoaming reagent selected from the group consisting of (a) solublelignin sulfonate salts and (b) dextrin and sulfonated nigrosines (c) andmixtures thereof, and (2) treating the resulting defoamed, coalfine-containingslurry with a reagent essentially comprising (a) 5 to100% of a water soluble polymer of molecular weight greater than100,000, selected from the group consisting of polyaerylamides,hydrolyzed polyacrylonitrile's, copolymers of acrylamide and acrylicacid, and the salts of copolymers of vinyl acetate and maleic anhydride,and (b) 095% of pregelatinized starch.

2. A process of recovering floating coal fines from aqueous slu rriestopped with coal fine-containing-froths which comp-rises the successivesteps of (1) adding to said slurries about 2.5 to 10 parts per 1,000,000of a do foaming reagent essentially comprising a mixture of crudecalcium lignin sulfonate, 29.4% corn dextrin and 0.6% sulfonatednigrosine; and (2) treating the resulting defoamed coalfine-containing-slurry with a flocculating agent essentially comprisingone part of po-lyaorylamide of about 2,000,000 molecular weight and 10parts of pregelatinized starch.

References Cited in the file of this patent UNITED STATES PATENTS2,149,748 Samuel Mar. 7, 1939 2,266,954 Bonnet et a1 Dec. 23, 19412,937,143 Goren May'l7, 1960 FOREIGN PATENTS 515,592 Great Britain Dec.-8, 1939 477,437 Canada Oct. 2, 1951.

200,883 Australia Feb. 15, 1956.

OTHER REFERENCES Hackhs Chemical Dictionary, McGraw-Hill Book Company,Inc., NY. (1944), page 571.

- Separan 2610 in the Coal Industry, Dow Chemical Co. Tech ServiceBulletin, Midland, Michigan, Code N0. 164, 22 (June 1956).

2. A PROCESS OF RECOVERING FLOATING COAL FINES FROM AQUEOUS SLLURRIESTOPPED WITH COAL FINE-CONTAINING-FROTHS WHICH COMPRISES THE SUCCESSIVESTEPS OF (1) ADDING TO SAID SLURRIES ABOUT 2.5 TO 10 PARTS PER 1, 000,000 OF A DEFOAMING REAGENT ESSENTIALLY COMPRISING A MIXTURE OF 70% CRUDECALCIUM LINGIN SULFONATE,
 29. 4% CORN DEXTRIN AND 0.6% SULFOANTEDNIGROSINE; AND (2) TREATING THE RESULTING DEFOAMED COALFINE-CONTAINING-SLURRY WITH A FLOCCULATING AGENT ESSENTIALLY COMPRISINGONE PART OF POLYACRYLAMIDE OF ABOUT 2, 000, 000 MOLECULAR WEIGHT AND 10PARTS OF PREGELATINIZED STARCH.